Quicksand is often exaggerated in popular culture, frequently depicted as a bottomless pit capable of swallowing a person whole. This dramatic portrayal has created a significant misunderstanding about its true nature and the physics governing its behavior. The question of whether quicksand is a non-Newtonian fluid is central to understanding its unique characteristics. Answering this requires a basic knowledge of fluid mechanics and how materials respond to external forces.
Defining Non-Newtonian Fluids
Viscosity is a fluid’s resistance to flow, such as the difference between water and honey. Newtonian fluids, like water or gasoline, exhibit a constant viscosity regardless of the shear stress or force applied to them. If water is stirred gently or vigorously, its resistance to flow remains the same.
Non-Newtonian fluids defy this predictable relationship because their viscosity changes when shear stress is applied. This category includes two types: shear-thickening and shear-thinning fluids. Shear-thickening fluids, such as a cornstarch and water mixture (oobleck), become thicker and more solid when hit quickly. Conversely, shear-thinning fluids, such as ketchup, become thinner and flow more easily when shaken or stirred.
The Formation and Composition of Quicksand
Quicksand is not a special type of soil, but a temporary condition of ordinary sand, silt, or clay. It forms when a saturated suspension of granular material is created, often due to an upward flow of water from an underground source. This upward pressure reduces the friction between the sand grains, which normally supports weight.
The sand grains are kept in a loosely packed state, floating within the water instead of locking together. This mixture is considered a colloid hydrogel, where fine solid particles are suspended in a liquid. When water cannot escape the saturated area, the resulting liquefied soil loses its strength and can no longer support the weight of an object.
Quicksand’s Behavior Under Stress
Quicksand is a non-Newtonian fluid, specifically a shear-thinning material. When left undisturbed, quicksand appears solid and slowly increases in viscosity over time. However, applying even a small amount of external stress, such as a person stepping onto it, causes an immediate decrease in its apparent viscosity.
This behavior is defined by its yield stress, the minimum force required to break its solid-like structure and make it flow. Quicksand’s structure is fragile; when pressure is applied, the grains momentarily lose contact, trapping the water and causing liquefaction. This sudden change is described as thixotropic behavior, meaning the material takes time to return to its original high-viscosity state after being disturbed. The continuous struggle of a trapped person further decreases the viscosity around their submerged body, causing them to sink deeper into the liquefied mixture.
Density and the Myth of Sinking
The depiction of total submersion in quicksand is a misconception. Quicksand is a mixture of water and dense particles, giving it a density of approximately 2 grams per cubic centimeter. The average density of the human body is only about 1 gram per cubic centimeter.
Because a person is less dense than the quicksand, it is impossible to sink completely according to Archimedes’ principle of buoyancy. A person entering quicksand sinks only until their body displaces a volume of the quicksand mixture equal to their own weight. This level is typically around the waist or chest, leaving a significant portion of the body above the surface. The real danger comes from the difficulty of extraction, as the quicksand quickly re-solidifies into its high-viscosity state when movement stops, effectively trapping the body.